Heat-sensitive recording can be conducted by simple recording devices, has high reliability and required little maintenance. Accordingly, heat-sensitive recording techniques have been continuously developed in recent years. Heat-sensitive recording can be adapted to various applications by utilizing the advantages thereof. However, it is considered that the reproduction of images by heat-sensitive recording is difficult as compared to electrophotography and ink jet recording. Conventional systems for obtaining a multi-color image by heat-sensitive recording include a thermal transfer system wherein a film coated with dyes is placed on an image receiving sheet, and the laminate is heated to transfer the dyes onto the sheet; and a sublimation transfer system. In these heat-sensitive recording systems, the recording devices are complicated because transfer sheets are used to transfer the dyes. Furthermore, at least three transfer sheets must be used to obtain one sheet of a full color image. Accordingly, the above-described recording systems cannot make the best use of the advantages of heat-sensitive recording and are disadvantageous in terms of cost. Attempts have long been made to heat directly the heat-sensitive recording materials to form colors; namely, to reproduce many colors by direct heat-sensitive recording.
JP-B-49-69 (the term "JP-B" as used herein means an "examined Japanese patent publication") proposed a heat-sensitive recording material Containing a plurality of electron donating dye precursors having different color formation initiating temperatures and a plurality of electron accepting compounds. Different temperatures are applied to thereby utilize the different color formation initiating temperatures of the dye precursors to obtain an image having various hues. Furthermore, JP-B-49-27708 and JP-B-51-5792 proposed a heat-sensitive recording material comprising two heat-sensitive color forming layers providing different hues in a laminated form, where the upper layer forms a color at a lower temperature, and both the upper layer and the lower layer form colors at a higher temperature to thereby form two colors. JP-B-51-5791 proposed a multi-color heat-sensitive recording material comprising a support having thereon a first heat-sensitive color forming layer containing a diazonium salt compound and a coupler, an interlayer containing a polyether compound and a second heat-sensitive color forming layer containing a basic dye precursor and an electron accepting compound in this order in a laminated form. In this arrangement, the second heat-sensitive color forming layer forms a color at a lower temperature, and the polyether compound erases the color of the second heat-sensitive color forming layer at a higher temperature. At the same time, the first heat-sensitive color forming layer forms a color, to thereby obtain a two-color image. The feature of JP-B-51-5791 resides in that the color of the second heat-sensitive color forming layer is erased during color formation of the first heat-sensitive color forming layer, to thereby obtain a color fraction having no color mixture.
JP-B-51-29024 proposed a two-color heat-sensitive recording material having two heat-sensitive color forming layers comprising basic dye precursors and electron accepting compounds in a laminated form, where guanidines (organic base compounds) are added to the lower color forming layer to erase the color of the lower-temperature color forming layer during color formation of the higher-temperature color forming layer. Furthermore, JP-B-51-37542-proposed a multi-color heat-sensitive recording material comprising a support having thereon a higher-temperature heat-sensitive color forming layer comprising an acid dye precursor and an organic base compound and a lower-temperature heat-sensitive color forming layer comprising a basic dye precursor and an electron accepting compound in a laminated form, where the organic base compound contained in the lower layer is diffused in the upper layer during high-temperature printing to thereby erase the developed color of the color former.
The above-described multi-color heat-sensitive recording materials are designed to reproduce two or more colors. However, even after considerable investigative effort, these systems are still ineffective in reproducing a full color image which has been in commercial demand in recent years.
As a method for reproducing a full color image by direct heat-sensitive recording, a heat-sensitive recording material is known comprising two heat-sensitive recording layers containing a combination of two kinds of diazonium salts having different light-sensitive wavelengths with couplers capable of reacting with the diazonium salts by heating to form different hues, and a heat-sensitive recording layer containing a combination of a basic dye precursor with an electron accepting compound in a laminated form of these layers, to thereby obtain a good multi-color image. This recording material comprises a support having laminated thereon a first heat-sensitive color forming layer comprising an electron donating dye precursor and an electron accepting compound, a second heat-sensitive color forming layer containing a diazonium salt compound having a maximum absorption wavelength of 360.+-.20 nm and a coupler capable of reacting with the diazonium salt compound by heating to form a color, and a third heat-sensitive color forming layer containing a diazonium salt compound having a maximum absorption wavelength of 400.+-.20 nm and a coupler capable of reacting with the diazonium salt compound by heating to form a color.
In a recording method using the above-described multi-color recording material, heat is first applied to third heat-sensitive recording layer 4 of FIG. 1 to form a color by reaction of the diazonium salt with the coupler contained in the same layer; the recording material is then irradiated with light of 400.+-.20 nm to decompose the diazonium salt contained in the third heat-sensitive recording layer; and a sufficient amount of heat is applied to the recording material so that the recording of second heat-sensitive recording layer 3 of FIG. 1 can be conducted to form a color by reaction of the diazonium salt with the coupler contained in the same layer. In this case, high-intensity heat energy is applied to the third heat-sensitive recording layer. However, since the diazonium salt of the third layer is decomposed and loses color formability, no color is formed. Subsequently, the recording material is irradiated with light of 360.+-.20 nm to decompose the diazonium salt contained in the second heat-sensitive recording layer, and a sufficient amount of heat is applied to the recording material so that the recording of first heat-sensitive recording layer 2 can be conducted to form a color. In this case, high-intensity heat energy is applied to the second and third heat-sensitive recording layers. However, since the diazonium salt in the second and third layers is decomposed and loses color formability, no color is formed.
In the above-described recording method, a full color image can be recorded when the compounds contained therein are selected so that the developed hues of the first, second and third heat-sensitive recording layers provide the primary three colors of yellow, magenta and cyan colors in a subtractive mixture, respectively.
The compounds used in the second and third heat-sensitive color forming layers include diazonium salt compounds, couplers capable of reacting with the diazonium salt compounds to form colors and basic materials capable of accelerating the reaction of the diazonium salt compounds with the couplers. These compounds are conventionally used in the field of diazo type copying paper and fixing type heat-sensitive recording papers using diazo compounds. Many compounds are conventionally known.
However, the recording materials containing diazo compounds have a disadvantages in that pre-coupling (a color reaction) gradually proceeds during storage before recording, and undesired coloring (fog) tends to occur. Various attempts have been made to solve the problem of coloring (fog). For example, JP-A-57-123086 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposed to use any one of ingredients participating in the color reaction in the form of discontinuous particles (e.g., a solid dispersion) or separated as a separate layer to prevent the ingredients from being brought into contact with each other, to thereby prevent pre-coupling from proceeding. The preservability (raw preservability) of the recording materials is considerably improved by the above-described means. However, the thermal response property (fusibility by heat), which is an important performance criteria, tends to deteriorate. Furthermore, it is known that color forming components can be separated from one another by encapsulating one of the components in a non-polar waxy material (see, JP-A-57-44141, JP-A-57-142636) or a hydrophobic high-molecular (weight) material (see, JP-A-57-192944) to improve raw preservability as well as the thermal property.
In these encapsulating methods, the waxy materials or the high-molecular materials are dissolved in an appropriate solvent, and the color forming component is dissolved or dispersed in the resulting solution to form capsules. Accordingly, the capsules are different in conception from ordinary capsules where a core material is surrounded by a shell. When the color forming component is dissolved to form the capsules, a problem can arise in that the color forming component is not always processed into a core material and instead is uniformly mixed with the encapsulating material. Consequently, pre-coupling gradually proceeds at the interface between the shells of the capsules, and good raw preservability cannot be maintained. On the other hand, when the color forming component is dispersed to form the capsules, thermal response property is lowered since a color reaction does not take place when the shells of the capsules are not molten by heat. In addition thereto, the solvents used for dissolving the waxy materials or the high-molecular materials disadvantageously must be removed after the capsules are formed.
To solve these problems, a method has been proposed where at least one member of components which participate in the color reaction is contained in the core material, and the core material is surrounded with a shell formed by polymerization to thereby microencapsulate the member (see, JP-A-59-190886, JP-A-60-6493). When color recording is conducted after conventional heat-sensitive recording materials are stored under high temperature conditions for a long period of time, the image density thus obtained is often lower than that obtained before storage. Accordingly, there is a need in the art to improve storage stability. It is known that density of the developed color image is lowered when the diazo compounds contained in the color forming recording layer are decomposed by heat and, as a result, the amounts of the diazo compounds are reduced. It is also known that diazo compounds are generally unstable compounds. The stability of the diazo compounds depends on their crystal state and solution state. Conventional methods of increasing the thermal stability of diazo compounds include a method where solvated compounds are formed by water; a method where double salts are formed; a method where strong acid anions are used; and a method where acid compounds are added.
When high-purity diazo compounds are desired, a purification operation such as recrystallization is often conducted. In these cases, stabilization by solvation or the use of additives cannot be expected. In some cases, the diazo compounds are dissolved in organic solvents depending on the intended purpose, but the organic solvents are incapable of stabilizing the diazo compounds by solvation.
The thermal stability of the diazo compounds is necessarily increased for the above noted purpose. The method for increasing stability of the diazo compounds by adding an acid compound is not always effective when a base is present (in accordance with a preferred embodiment of this invention).
The present inventors have extensively studied on the thermal stability requirements of diazo compounds. As a result, the present inventors have discovered that compounds having a specific diazonium ion structure have excellent thermal stability.
Recording materials having excellent preservability before recording (hereinafter referred to as raw preservability) could be obtained by using the compounds disclosed in JP-A-1-80588.
However, the compounds disclosed in JP-A-1-80588 are disadvantageous in that sufficient fixing cannot be effected by light of 420.+-.20 nm used for fixing (i.e., the operation where undesired diazonium compounds left behind after thermal recording are decomposed by light to deactivate the same) diazonium compounds contained in the third heat-sensitive recording layer.
To solve the above problem, the compounds disclosed in JP-A-4-59287 are developed. When these compounds are used, recording materials having excellent fixing properties and excellent raw preservability can be obtained. Furthermore, couplers capable of providing yellow dye images in combination with the compounds disclosed in JP-A-4-59287 are disclosed in JP-A-4-201483.
However, yellow dye images obtained from these combinations are disadvantageous in that fastness to light is poor. Furthermore, unfavorable coloring occurs after the diazonium salt compounds are decomposed by light.